This invention relates to cutting apparatus, more particularly, though not exclusively, to a cutting apparatus for cutting a workpiece in the form of a pipe or a tube. The invention also relates to a method for cutting a workpiece such as a pipe or a tube, and to a method of joining a pipe to a host pipe.
It is often necessary to cut a hole or a `window` in the wall of a pipe or a tube, for instance when joining a branch pipe to a host pipe to form a T junction. In most cases it is desirable to cut the window as accurately as possible, both in terms of depth of cut and line of cut. For example, pipes of cast iron or other metal are often lined with a plastics liner and, when connecting a branch pipe to the host pipe, it is conventional to seal the branch pipe to the plastics liner. It is, therefore, desirable to cut out a window from the metal outer wall without penetrating or without significantly cutting into the plastics liner, thereby exposing an area of the outer surface of the plastics liner to which the branch pipe can be connected in a subsequent operation. It will be appreciated that precise control over the depth of cut can be particularly important, so as to ensure that the metal outer wall is substantially fully penetrated without or without, significantly, cutting into the plastics liner.
Unfortunately, accuracy has been found to be difficult to achieve in practice for a number of reasons. For example, the curved outer surface of the pipe makes it sifficult to establish, and to maintain, a reference surface against which cutter movement can be measured. Another difficulty arises from the location of the pipe itself; it is usual for pipes to be worked on in situ and access to the pipe may well be restricted, particularly where the pipe is underground such as in a gas main or a water main system.